This post is a working draft; Target Atmospheric Methane should be more useful.
Yesterday I submitted
a question to James Hansen; Robert Goodland; Ivar Ivaksen; Joe Romm (and his readership); and a number of other trusted advisors in the climate change community. This morning, I heard back from
Keith Akers, who had a lot of good questions to ask. In response I, prepared a second draft as follows (then corrected this draft two more times to eliminate further errors):
1. Given a current forcing of ~1.5 w/m2 for CO2 and ~0.5 w/m2 for CH4, it looks like CO2 is now responsible for 3x as much warming as CH4.
2. But this is clearly not correct from a policy perspective, because when we talk about “global warming,” we are not interested
exclusively in total warming, but we are interested especially in the margin of forcing that exceeds a desirable steady-state “greenhouse” baseline.
3. An increase from a preindustrial CO2 level of 280 ppm to a current CO2 level of 392 ppm is an increase of 112 ppm. The increase from a preindustrial level of CH4 at 750 ppb to a current level of around 1800 ppb is an increase of 1050 ppb.
4. If we now restrict our attention to current CO2 versus CH4 warming in relation to these preindustrial greenhouse levels, we get an “excess forcing” for CO2 of ~0.43 w/m2 for CO2 (112/392 x 1.5 w/m2), and an “excess forcing” of ~0.29 w/m2 for CH4 (1050/1800 x 0.5 w/m2). In other words, the current CO2 warming is only 1/3 greater than the CH4 warming in terms of a preindustrial greenhouse baseline.
[Detail: If 392/392 ppm CO2 = 1.5 w/m2 of forcing, than 112/392 ppm of CO2 = x/1.5 w/m2 of forcing; solve for x. The same logic applies to methane in ppb. The point is to determine what portion of the current widely accepted forcing is attributable to the amount of atmospheric gas that is now above the preindustrial level].
5. 4. No prominent climate stabilization negotiator that I am aware of is recommending a return to preindustrial levels. If, then, we restrain ourselves to the most ambitious but practically achievable CO2 climate stabilization target that is widely promoted for policy consideration - Hansen et al's 350 ppm CO2 target - than it is reasonable to recalibrate both CO2 and CH4 forcing to this proposed new steady-state climate regime. Insofar as 350 ppm CO2 is a 25% increase in the preindustrial level of 280 ppm, a proportional 25% increase in CH4 over the preindustrial level of 750 ppb is obtained at 940 ppb CH4.
It seems reasonable to assume a proportional increase in CH4 if our goal is temperature stability, since temperature in the Vostok ice core record tracks almost perfectly with the curve of all GHGs in the aggregate. [I could aim for less than 940 ppb CH4 with an eye toward cooling, but I here want to determine an upper methane bound within which humanity should try to restrict itself, consistent with Hansen et al's target of 350 ppm for CO2.]
How do the current CO2 and CH4 forcing compare to these more realistic steady-state targets? The forcing for current 392 ppm CO2 over 350 ppm is ~0.16 w/m2 (42/392 x 1.5 w/m2), and for 1800ppb CH4 over 940 ppb it is ~0.24 w/m2 (860/1800 x 0.5 w/m2).
5. If we now restrict our attention to current CO2 versus CH4 warming in relation to these more practical steady state targets, we get an “excess forcing” of ~0.56 w/m2 for CO2 (42/112 x 1.5 w/m2), and an “excess forcing” of ~
0.50 0.41 w/m2 for CH4 (860/
8601050 x 0.5 w/m2). In other words, the current CO2 warming is just
a bit 0.15 w/m2 more than the CH4 warming in terms of a practical steady state greenhouse baseline.
[Detail: If 112/112 ppm CO2 = 1.5 w/m2 of forcing, than 42/112 ppm of CO2 = x/1.5 w/m2 of forcing; solve for x. The same logic applies to methane in ppb. The value of 42 = 392-350 ppm CO2. The value of 860 = 1800-940 ppb CH4. The point is to determine what portion of the current widely accepted forcing is attributable to the amount of atmospheric gas that is now above the practical steady state level. A climate physicist will calculate this value in a more accurate manner. As noted later, the time horizon is a critical variable].
This A value of ~0.5 w/m2 for methane is consistent with NOAA's
description of Mauna Loa In Situ Methane Observations: "Direct radiative forcing due to the increase in methane
since pre-industrial times is ~0.5 Wm -2." Approximately 81% of this forcing exceeds a post-industrial CH4 cap of 940 ppb that is proportional to a postindustrial CO2 cap of 350 ppm.
6. According to the
April 2011 paper by Dr. Isaksen, et al., the indirect CH4 radiative forcing is equal to its direct forcing when effects on O3, H2O and CH4 half-life are taken into account. This, on top of point #5, would give current CH4 a forcing of
~0.48 w/m2, ~1.0 ~0.82 w/m2, or
3x ~2x ~1.5x greater than the proportional current CO2 forcing of
~0.16 w/m2 ~0.56 w/m2.
7. Finally, while the radiative forcing of CO2 emissions is significantly offset by the negative forcing of reflective aerosols created during the combustion of fossil fuels, there are no aerosols associated with methane, which makes the effects of CH4 even greater. This could reduce the CO2 warming effect by 1/2 (or more), thereby again doubling the relative CH4 warming effect to
as 6x. as much 4x 3x.
I am going to leave the argument at that, and hold the GWP discussion as a separate issue, until I get confirmation from a physicist familiar with GWP and radiative forcing. As I look at the formulae presented on the
appropriate Wikipedia page,
and think practically about the issue, it seems impossible to calculate radiative forcing [moving forward] without reference to a time horizon. If the CH4 forcing relative to CO2 at 100 years is 25, and this is the value on which the widely cited CO2 to CH4 forcing values depend, the methane GWP is 49 at year 50 and 72 at year 25, and look at
this useful paper,
Either of these adjustments - and there are good arguments to make the 50-year adjustment at the least - would therefore potentially further increase the CH4 forcing noted above to 12x or 18x 8x to 12x 6x to 8x the marginal I am convinced that any prospective consideration of CO2 forcing over the next 20-40 years should take into account the true comparative forcing (positive or negative) of changes in CH4 forcing over the same time period.
It seems clear to me that even with these recalibrations, the basic structure and result of the argument remains very much the same. I know there must be something so profoundly wrong with it, that a climate physicist should be able to identify and correct the problem fairly easily. I await said correction(s).
Later, I may have time to share an extensive description of how I came to formulate this particular sequence of intellectual operations at this particular point in time; how it relates to a three-part core structure for an international climate stabilization treaty; and what the implications are for short-term and mid-term action. For a preliminary discussion tailored to the local level,
an article I wrote yesterday on the regulation of livestock slaughter may be a good starting point.
I will be returning to blogging ahead of schedule and retargeting my life efforts in light of this new development.
First Reflection on 9/19/2011 Update at 9:21 PM
This update started out as
a comment on Joe Romm's blog:
Okay, I found the flaw. The forcing values for carbon 1.5 w/m2 and 0.5 w/m2 methane HAVE to be the forcings just from the preindustrial baseline to present. That’s my mistake, in the very first step. Of course that has already been thought of. I couldn’t possibly feel more relieved and humiliated at the same time! At most, considering direct and indirect effects, a shorter time horizon, and CO2 reflective aerosol offsets, the methane impact is perhaps 2-3x greater than CO2. Is that correct?
However, when I then came back to this page and recalibrated everything (
yet again several times) per the above (and other) corrections, I was surprised to discover that end result doesn't look that much different!
Second Reflection on 9/19/2011 Update at 9:21 PM
I should no doubt simply throw in the towel and leave the matter to better trained and more qualified minds - with less of a
vegan bias, perhaps - but I want to understand what happens when I advance my thought experiment forward to a projected 40-year climate stabilization target of climate-neutrality at 430 ppm CO2 by 2050, while holding CH4 completely steady at its current level of 1800 ppb.
Relative to a steady state target of 350 ppm for CO2, this will result in an “excess forcing” of ~1.07 w/m2 for CO2 (80/112 x 1.5 w/m2), and a direct “excess forcing” of ~
0.5 0.41 w/m2 for CH4 (860/
860 1050 x 0.5 w/m2). In other words, the future CO2 warming at 430 ppm in 2050 will then be double the direct CH4 warming in terms of a practical steady state greenhouse baseline.
But if the
indirect CH4 forcing is indeed a doubling; or the CO2 aerosol reflectivity offset is indeed a halving (or more); or the more pertinent CH4 GWP really is between 20 and 50 years; if any ONE of these conditions is true, than the CO2 forcing will
just then in 2050 just be
catching up to the exceeding the existing CH4 forcing now in 2011. If two or especially all three of these conditions prove true, however, than CO2 stabilization at 430 ppm by 2050 will not mean much
of anything at all compared to CH4
in relative terms. We need to start paying serious attention to methane now.
The UNFCCC requirement for precaution requires us to seriously ask ourselves whether we want to bet against all three of the above plausibilities in our assessment of the methane risk.
At the same time, keeping the Arctic summer sea ice albedo
and ice sinks intact - and keeping northern latitude methane
sinks [sources] in the ground [and water] - also seem like fairly good ideas. Given the uncertainties in our current location on a trajectory somewhere between a 350 ppm CO2 tipping level and an unknown future "non-linear point of no return," precaution would further seem to require that we do the one thing we know we can do ASAP, which is to take advantage of the short methane half-life and the adverse health effects of animal protein in the human diet to significantly cool the planet at the decadal scale with immediate and significant dietary transition.
It is absolutely absurd to risk the future of this planet on an atavistic meat and dairy fetish that we can all start changing tomorrow at zero real biophysical cost to our economies. Livestock reduction is a net energy and resource savings
all the way for the global economy, and it will in almost all cases create greater opportunities for CO2 sequestration through forestation.
[At this point, I still do not have a precise handle on just how much we could expect to reduce our methane emissions through dietary transition alone. This still needs to be carefully modeled. See Slide 22 in my 9-24-2011 slideshow.]
Reflection on 9/21/2011 Update
At some point, this post will probably be more strikeout than viable text, but in the meantime, so long as this sentence holds true - "It seems clear to me that even with these recalibrations, the basic structure and result of the argument remains very much the same" - I am going to let the post stand as a working document.
My father has been good enough to review all of my calculations, and last night he caught the hasty error by which I substituted 860 ppb rather 1050 ppb for the denominator in my calculation of proportional methane forcing.
My dad also sent me a quote from the introduction to
Methane Emission from Rice Fields. The quote is based on citations from the mid 1980s, and is useful to include here as the lead-in to another thought experiment:
Public concern about global warming mostly focuses on carbon dioxide, the most prevalent greenhouse gas. Methane (CH4), the major component of natural gas, is second in importance as a greenhouse gas. Methane concentration in the atmosphere has more than doubled during the last 200 years. Its current atmospheric concentration of 1.7 ppm by volume, up from 0.7 ppm in preindustrial times, is much lower than the 345 ppm of carbon dioxide, up from 275 ppm. But one molecule of methane traps approximately 30 times as much heat as does carbon dioxide. The heating effect of the atmospheric methane increase is approximately half that of the carbon dioxide increase (Dickinson and Cicerone 1986, Ramanathan et al. 1985). Continued increase in atmospheric methane concentrations at the current rate of approximately 1% per year is likely to contribute more to future climatic change than any other gas except carbon dioxide (Cicerone and Oremland 1988) and may significantly contribute to a negative feedback system with unpredictable consequences for the whole chemistry of the atmosphere.
This is a good summary of the conventional wisdom that has prevailed for the last twenty-five years. But it should now be easy to see how this conventional wisdom represents a systematic bias against a precautionary valuation of CH4 forcing relative to CO2 forcing.
The minimization of CH4 forcing to a mere 25x or 30x greater potency than CO2 is based on a 100-year time horizon for calculation of GWP that does not accurately reflect the relative degree of CH4 to CO2 forcing that has taken place on planet earth in the time since this paper was written. Simply adjusting the time horizon to fifty years increases the CH4 GWP to 50, while further reducing the time horizon to twenty years - an even more appropriate value - increases the CH4 GWP to 72. A GWP of 72 is 2.88x greater than a GWP of 25.
But the use of a 100-year time horizon for GWP doesn't just minimize our perception of CH4 forcing. It also minimizes our perception of the relative cooling effect we can achieve over decadal time scales, if we significantly contract our CH4 emissions, primarily through improved regulation of
health-injuring luxury commodities in the food and agriculture sector.
The bias against CH4 that is evident in this snapshot of the 1985 climate science paradigm does not stop there, however. The conventional wisdom also neglects indirect forcings that significantly raise the net warming value of CH4, on the one hand, and diminish the net forcing value of CO2, on the other. When these indirect forcings are taken into account, together with GWP, it seems clear that CH4 is responsible for more net warming than CO2
over the entire course of the post-industrial period.
The relative forcing of CH4 over CO2 is further magnified if we restrict our attention to a practical post-industrial target of about 350 ppm CO2, and match that with a proportional methane target of about 940 ppb CH4. But this recalibration in light of a new steady-state baseline target is clearly not the only way to make the point I am trying to make about the true importance of methane.
Consider another thought experiment. What happens if we imagine that in 1985 - when, per the above reference, CO2 was at 345 ppm and CH4 was at 1700 ppb - world leaders had agreed to "limit" CO2 emissions to 390 ppm by 2010, and to reduce CH4 emissions to 1500 ppb within the same time period through changes in the global food and agriculture system? What would that hypothetical scenario generate in terms of net forcing from CO2 and CH4 during the 25-year period, relative to the actual CO2 and CH4 forcing that took place during this time?
For actual CO2 forcing, I get a direct forcing of ~0.60 w/m2 (45/112 x 1.5 w/m2) for CO2 from 1985 to 2010. However, the net effect of this CO2 forcing must be tempered by the associated indirect negative forcing associated with reflective aerosols, which could reduce it by 50% or more, in real-world terms, to ~0.30 w/m2 or less. That is, over the last 25 years, the increase in atmospheric CO2 from 345 to 390 ppm has plausibly resulted in a net positive forcing of ~0.3 w/m2.
This is obviously a significant value, but how does it compare to the actual increase in positive forcing that took place as CH4 climbed from 1700 ppb to 1800 ppb during the same period?
I get an initial positive forcing of ~0.05 w/m2 for CH4 (100/1050 x 0.5 w/m2) using the conventional 100-year time horizon for CH4. However, if a more plausible 20-year time horizon for this experiment results in a CH4 forcing effect that is actually 2.88x greater, the positive forcing reduction is closer to ~0.14 w/m2 for this 25-year change in CH4, which is further doubled by indirect effects to ~0.28 w/m2. This brings the plausible incremental gain in CH4 forcing so close to the plausible incremental gain in CO2 forcing that the entire question of which increase in atmospheric gas during the period is responsible for more net heating depends entirely on their indirect effects. To the extent that the negative forcing from reflective aerosols produced by the combustion of fossil fuels is actually greater than 50% of the CO2 positive forcing; or to the extent the CH4 indirect positive effect is greater than 100% of the CH4 direct positive forcing; the slight increase in CH4 concentrations may actually be responsible for more net warming during this period than the CO2!
What if, instead of this net gain in GHG forcing from CO2 and CH4 of approximately 0.6 w/m2 since 1985, the only thing humanity had done differently was reduce CH4 in the atmosphere by 200 ppb, from 1700 ppb to 1500 ppb?
The total CH4 difference in this case is from 1800 ppb currently to 1500 ppb under the hypothetical scenario, for a total reduction of 300 ppb. This returns an initial reduction in positive forcing of -0.14 w/m2 for CH4 (300/1050 x 0.5 w/m2) using the conventional 100-year time horizon for CH4. However, if a more plausible 20-year time horizon for this experiment results in a CH4 forcing effect that is actually 2.88x greater, the reduction in positive forcing is closer to -0.40 w/m2 for this 25-year change in CH4, which is further doubled by indirect effects to -0.80 w/m2. In other words, this change may have resulted in a real decrease of total post-industrial methane forcing of about -0.80 w/m2.
Since the real increase in total post-industrial net CO2 forcing during this same historical time period was plausibly about 0.3 w/m2, this suggests it was entirely plausible, within a time horizon of 25 years from 1985 to 2010, to have engineered a
net reduction in forcing from these two greenhouse gases of -0.5 w/m2, relative to their combined forcing today. Or, we could look at this as a negative forcing of -0.3 w/m2, relative to their combined net forcing effect in 1985.
Either way, the result is a
net GHG cooling relative to a new, practical, long-term climate stabilization target of around 350 ppm CO2.
This net GHG cooling would have been possible with practically no restrictions on CO2 emissions (as was this case), allowing atmospheric CO2 concentrations to rise all the way up to 390 ppm (as they did). It would have been made exclusively through a reduction in the consumption of health-impairing luxury livestock commodities.
